Dioon Edule Lindl., Zamiaceae) Is a Medium-Sized Cycad with an Erect Trunk and a Rigid Crown of Long, Light Green Leaves
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This article may be downloaded for personal use only. Any other use requires prior permission of the author or publisher. The following article appeared in Raymundo Mora, Laura Yáñez-Espinosa, Joel Flores, Nadya Nava-Zárate. Strobilus and Seed Production of Dioon Edule (Zamiaceae) in a Population with low Seedling Density in San Luis Potosí, Mexico. Tropical Conservation Science. Vol. 6(2):268-282. Copyright © 2013. SAGE Publications. And may be found at https://doi.org/10.1177/194008291300600208 Mongabay.com Open Access Journal - Tropical Conservation Science Vol.6 (2):268-282, 2013 Research Article Strobilus and seed production of Dioon edule (Zamiaceae) in a population with low seedling density in San Luis Potosí, Mexico Raymundo Mora1, Laura Yáñez-Espinosa1,2, Joel Flores3, and Nadya Nava-Zárate4 1Facultad de Ingeniería, Universidad Autónoma de San Luis Potosí. Av. Dr. Manuel Nava 8, Zona Universitaria Poniente, San Luis Potosí, S.L.P, México, C.P. 78290, e-mail: [email protected] 2Instituto de Investigación de Zonas Desérticas, Universidad Autónoma de San Luis Potosí. Altair 200, Col. Del Llano, San Luis Potosí, S.L.P., México, C.P. 78377, e-mail: [email protected] 3División de Ciencias Ambientales, Instituto Potosino de Investigación Científica y Tecnológica, A.C. Camino a la Presa San José 2055, Col. Lomas 4 Sección, San Luis Potosí, S.L.P., México, C.P. 78216, e-mail: [email protected] 4Facultad de Estomatología, Universidad Autónoma de San Luis Potosí. Av. Dr. Manuel Nava 2, Zona Universitaria Poniente, San Luis Potosí, S.L.P., México, C.P. 78290. e-mail: [email protected] Corresponding author: Laura Yáñez-Espinosa, e-mail: [email protected] Abstract. We describe strobilus and seed development in a Dioon edule (chamal, palma, dameu’) population characterized by low seedling and high adult tree density, in order to improve conservation decisions for this endangered cycad species. Female strobili required 16-17 months and male 4-5 months to develop. During this period 80% female and 100% male strobili were not damaged by herbivores. The method of cone analysis used to evaluate seed production of pines was modified for D. edule, providing actual yield of individual strobili compared to potential seed yield, explaining productivity in terms of seed efficiency, and identifying and quantifying the types of seed failures. Larger strobili have higher seed potential calculated as the number of seeds potentially produced in the fertile megasporophylls of each strobilus, total seeds actually produced, and seeds with an embryo (filled). Calculated average seed potential per strobilus was of 230.4 seeds. Using the X-ray technique, average seed traits were 100.2 full, 1.8 insect damaged, and 0.6 malformed. Seed efficiency (number of filled seeds as a percentage of the seed potential of each strobilus) of 42.5% indicated that major seed loss was attributable to abortive ovules and seeds, possibly due to ineffective pollination. Seedling survival in greenhouse conditions was 100% after one year. Low seed production affects the population structure and hinders its conservation. Keywords: Cycads, seed production, cone analysis, strobili, X-ray analysis. Resumen. Se describe el desarrollo de estróbilos y semillas en una población de Dioon edule (chamal, palma, dameu’) caracterizada por la densidad reducida de plántulas y elevada de plantas adultas, para tomar decisiones más acertadas sobre la conservación de esta especie amenazada. Los estróbilos femeninos requieren 16-17 meses y los masculinos 4-5 meses para desarrollarse. Durante este período, 80% de los estróbilos femeninos y 100% de los masculinos no fueron dañados por herbívoros. El método del análisis de conos usado para evaluar la producción de semillas de pino fue modificado para D. edule, informando de la producción real de semillas en cada estróbilo comparada con la producción potencial, explicando la productividad en términos de eficiencia de semilla e identificando y cuantificando los tipos de pérdidas. Los estróbilos más grandes tienen mayor potencial de semilla, calculado como el número de semillas producidas potencialmente en los megasporófilos fértiles de cada estróbilo, total de semillas efectivamente producidas y semillas con embrión (llenas). El promedio de semillas potenciales fue de 230.4 semillas. Utilizando la técnica de rayos X, el promedio de las variables de las semillas fueron 100.2 llenas, 1.8 dañadas por insectos y 0.6 malformadas. La eficiencia de semilla de 42.5% (proporción de semillas llenas en relación al potencial de semilla de cada estróbilo) indicó que la mayor pérdida de semilla es atribuible a los óvulos y semillas abortados, posiblemente debido a la inefectividad de la polinización. La sobrevivencia de las plántulas en invernadero fue de 100% después de un año. La baja producción de semilla afecta la estructura de la población y dificulta su conservación. Palabras clave. Cícadas, producción de semillas, análisis de conos, estróbilos, análisis con rayos X. Tropical Conservation Science | ISSN 1940-0829 | Tropicalconservationscience.org 268 Mongabay.com Open Access Journal - Tropical Conservation Science Vol.6 (2):268-282, 2013 Received: 26 October 2012; Accepted: 9 May 2013; Published: 24 June 2013. Copyright: © Raymundo Mora, Laura Yáñez-Espinosa, Joel Flores and Nadya Nava-Zárate. This is an open access paper. We use the Creative Commons Attribution 3.0 license http://creativecommons.org/licenses/by/3.0/ - The license permits any user to download, print out, extract, archive, and distribute the article, so long as appropriate credit is given to the authors and source of the work. The license ensures that the published article will be as widely available as possible and that the article can be included in any scientific archive. Open Access authors retain the copyrights of their papers. Open access is a property of individual works, not necessarily journals or publishers. Cite this paper as: Mora, R., Yáñez-Espinosa, L., Flores, J. and Nava-Zárate, N. 2013. Strobili and seed production of Dioon edule (Zamiaceae) in a population with low seedlings density in San Luis Potosí, Mexico. Tropical Conservation Science Vol. 6(2):268-282. Available online: www.tropicalconservationscience.org Introduction The chamal or dameu’ (Dioon edule Lindl., Zamiaceae) is a medium-sized cycad with an erect trunk and a rigid crown of long, light green leaves. It is endemic to Mexico, distributed along the Sierra Madre Oriental from southern Tamaulipas, through San Luis Potosí, Querétaro, Hidalgo and to central Veracruz [1]. Extensive collection of the plants and destruction of their habitat threaten their biological viability, and Dioon edule is therefore listed as an endangered species by the Mexican government [2] and as near threatened by IUCN [3]. These plants are important from an evolutionary standpoint because they belong to the group of gymnosperms [4], i.e., their reproductive structures are strobili or cones. They are strictly dioecious: when they mature, they produce male or female strobili. The basic structure is similar in both males and females, with a central axis and structures called sporophylls or scales [5]. Although female plants produce one strobilus each year, male plants can produce from one to four. The female strobili are larger than the males and bear the ovules that subsequently will be fertilized by pollen. Studies conducted in populations of Dioon edule have shown that seed production is good, from 86 to 230 seeds per strobilus with75-90% germination [6, 7]. However, under natural conditions, the mortality at the seed and seedling stages is high (almost 90 %) [7] due to prolonged drought in the dry tropical forest habitat of D. edule, the recalcitrant nature of the seeds, predation by the field mouse Peromyscus mexicanus, and dehydration of seedlings during the first year [6-8]. Particularly in the region of the Sierra Madre Oriental in the state of San Luis Potosi, some populations of D. edule have densities of 775 to 3775 individuals ha-1 [9] and are currently threatened by prolonged drought, herbivory, fire, changes in land use, plunder of plants and seeds for illegal sale, and collection of the female strobili by some Xi‘iuy indigenous communities who use the seeds in food preparation [10]. The differences in population density may be due to the number of germinated seeds and established seedlings in each population. A mortality rate of 86 to 98% was observed by counting the number of plants producing female strobili in each population and considering the average number of potential seeds produced in each strobilus [9]. Nevertheless, one particular population presented the lowest seedling density (112 ha-1), 86% below the average (950 ha-1), in contrast to the highest reproductive adult density (250 ha-1), 66% above average [9]. We examined whether the low seedling density could be affected by low seed production and if so, what factors would be involved. Various methods for estimating seed production have been developed, considering that annual seed production in plant species is variable and can be attributed to several factors. Particularly in conifers, Tropical Conservation Science | ISSN 1940-0829 | Tropicalconservationscience.org 269 Mongabay.com Open Access Journal - Tropical Conservation Science Vol.6 (2):268-282, 2013 cone or strobilus analysis developed by Bramlett et al. [11] is useful because it is one of the most efficient methods; it compares the actual seed production with the potential production of an individual strobilus, allowing productivity to be expressed in terms of seed efficiency of the strobili that reach maturity. This method has already been used to evaluate seed production and the magnitude of its loss in different coniferous species, and to identify the factors that affect their production [12]. A basic knowledge of strobilus and seed development and morphology is essential to understand this method of strobilus analysis [11]. There are several studies of cycads distributed in Mexico, describing strobilus and seed morphology [5, 7, 13], or including some aspects of strobilus phenology and seed production [6, 14] but few describing seed anatomy [15-17].